In the mammalian lung, two types of pulmonary receptors (juxtapulmonary and slowly adapting pulmonary stretch receptors) have been reported to be sensitive to carbon dioxide. Recent reflex studies suggest that a third population of receptors exists in the mammalian lung which are equisitively sensitive to carbon dioxide and play a significant role in ventilatory control. RESEARCH OBJECTIVES: The objectives of this proposal are to (1) determine the response characteristics of pulmonary receptors to carbon dioxide in the ventilatory gas and in the mixed venous blood, (2) to determine if a population of CO2-sensitive receptors exists in the mammalian lung which are similar to the avian intrapulmonary chemoreceptors and (3) to determine the functional location of these receptors within the lung. METHODS: Electrophysiological techniques will be utilized. These techniques involve the use of tungsten microelectrodes to record extracellular single unit activity from vagal pulmonary afferent neurons located in the nodose gaglion in cats.